Study on lightening of coal tar with metal oxide supported γ-Al2O3 catalyst

In this experiment, a fixed bed of pyrolysis was used to conduct pyrolysis with coal and a mixture of coal and catalyst, and the distribution and composition of tar products were studied. The pyrolysis of raw coal was carried out at different temperatures and at different constant temperature times, and the effects of pyrolysis temperature and constant temperature pyrolysis time on tar product formation from raw coal pyrolysis were studied. γ-Al2O3 was used as the carrier, and 4 kinds of alkaline earth metal oxides (MgO, CaO, SrO, BaO), 3 kinds of subgroup metal oxides (Fe2O3, Co2O3, NiO) and 5 kinds of VIII metal oxides (Cr2O3, MnO2, CuO, ZnO, MnO2) were selected as active components. The supported γ-Al2O3 catalyst was prepared by the method of equal volume impregnation and roasting in a muffle furnace. The γ-Al2O3 catalyst was characterized by means of XPS, BET and SEM, and the mechanism of the mixed pyrolysis of coal with different metal oxide supported catalysts to generate tar was studied. The results showed that: (1) under the conditions of 450 °C, 500 °C, 550 °C and 600 °C, the maximum tar yield was 0.32 g at 600 °C, and the tar yield was higher at constant temperature for 15 min than at final temperature of 600 °C, with an increase of 15.63%. (2) Fe2O3/γ-Al2O3 catalyst resulted in the highest tar yield of 0.75 g, which was 134.38% higher than that of coal pyrolysis. (3) From the increase of light oil and phenol oil and the decrease of anthracene oil and asphalt, Co2O3/γ-Al2O3, Fe2O3/γ-Al2O3 and Cr2O3/γ-Al2O3 can improve the tar quality better.

There are many studies on the preparation of supported catalysts using metal oxides as coal pyrolysis cracking catalysts.CoO is loaded on coal semi-coke to prepare a catalyst for tar cracking to crack coal pyrolysis products, which increases the mass yield and content of light components in tar by 8.8% and 28.8%, respectively 7 .Liu cracked Pingshuo coal rapid pyrolysis tar after loading Mo into HZSM-5, which greatly increased the amount of benzene, toluene, ethylbenzene, xylene and naphthalene in the tar product 8 .Liao 9 selected four metal oxides (CaO, Al 2 O 3 , Fe 2 O 3 , NiO) as catalysts for pyrolysis of Huolinhe lignite.The catalysts have different effects on different pyrolysis stages of coal, and they can promote the conversion rate of lignite pyrolysis.The order of action is: NiO > CaO > Fe 2 O 3 > Al 2 O 3 ; the catalytic effect on lignite in the active pyrolysis stage is: NiO > Fe 2 O 3 > CaO > Al 2 O 3 , and the catalytic effect on lignite in the thermal polycondensation stage is: Fe 2 O 3 > CaO > NiO > Al 2 O 3 .Wang 10 used Co-Mo/Al 2 O 3 and Ni-Mo/Al 2 O 3 to catalyze coal pyrolysis, which has a significant effect on improving the lightening of pyrolysis tar, and can increase the yield of products such as light aromatics and naphthalenes in tar, so that the relative mass fraction of light aromatics reaches 40.94% to 50.89%.Cui 11 used MgO, Fe 2 O 3 , Co 2 O 3 , ZnO, NiO oxide-supported Al 2 O 3 catalysts to influence the pyrolysis gas products of Huangling coal, and Al 2 O 3 was used as a carrier to have good mechanical properties and catalytic performance.It can be seen from the above studies that metal oxides have good effects on coal pyrolysis and cracking, and can be used as active components of supported catalysts.Al 2 O 3 has become an excellent catalyst carrier due to its good physical and mechanical properties.At present, there were many researches on coal pyrolysis to prepare tar at home and abroad.Most of the tar was prepared and modified by pyrolysis/catalysis.It was also common to study the utilization of supported metal oxide catalysts for coal pyrolysis to prepare tar.However, the research on coal pyrolysis to tar was lack of metal oxide supported catalysts of different groups.Therefore, a series of metal oxide-supported catalysts for coal pyrolysis to prepare tar were proposed in this study, which is an innovative study.
Therefore, in this study, a fixed-bed pyrolysis reactor was used to prepare metal oxide-supported γ-Al 2 O 3 catalysts by muffle furnace calcination.The catalyst and coal are mixed and pyrolyzed to control the tar and improve the quality.It will provide a theoretical and practical basis for the comprehensive utilization of coal pyrolysis product resources, and further develop coal clean technology.

Experiment Experimental materials
The coal sample used in this paper is produced in Cuimu mining area of Shaanxi Province.The coal sample is broken up to 3 mm in particle size.The catalyst used in the experiment was a spherical γ-Al 2 O 3 with a particle size of 3 mm purchased on the market.

Preparation of catalyst
Preparation of alkaline earth metal oxide catalyst supported on γ-Al 2 O 3 : The equal volume impregnation method was used [12][13][14] .3g γ-Al 2 O 3 was immersed in 5% MgCl 2 solution for 24 h and then removed and placed in a muffle oven at 500 °C for 4 h.The MgO/γ-Al 2 O 3 catalyst with 5% loading was removed after cooling to room temperature.CaO/γ-Al 2 O 3 , SrO/γ-Al 2 O 3 and BaO/γ-Al 2 O 3 catalysts were prepared by the same method.
Preparation of subgroup metal oxide catalyst supported on γ-Al 2 O 3 : the equal volume impregnation method was used.3 g γ-Al 2 O 3 was immersed in 5% Fe(NO 3 ) 3 solution for 24 h and then removed and placed in a muffle oven at 500 °C for 4 h.The Fe

Experimental evaluation
The catalytic cracking process of coal tar is shown in Fig. 1.
The 10 g coal sample and the 3 g catalyst were mixed evenly and placed in the pyrolysis furnace.During the experiment, the temperature of catalyst and coal were from 24 to 600 °C, and the temperature was constant at 600 °C for a period of time.With the increase of temperature, the gaseous producted from coal pyrolysis to the tar collection bottle, and the tar components and the tar produced was analyzed by GC.

Tar detection
The detection of coal tar is mainly carried out by means of simulated distillation to analyze the distribution of each fraction in the tar, and is carried out on a simulated distillation chromatogram.The principle of this method is a non-polar column with a certain degree of separation, testing the retention time of known mixture components under linear temperature programming conditions.Then, under the same chromatographic conditions, the samples are sequentially separated according to the boiling points of the components, and the slice integration is performed to obtain the corresponding cumulative area, and the corresponding retention time.After temperature-time interpolation correction, a temperature corresponding to a percent yield, that is, a distillation range, is obtained, wherein the cumulative area percentage is the yield.Table1 shows the fractions and corresponding boiling points of simulated distillation gas chromatography plus coal tar measurement.The simulated distillation experiment was used to determine the tar components in this experiment.The dewatered tar samples were filtered in acetone solvent, and the prepared tar samples were prepared.Set operating conditions, injection mode: on-column, gasification temperature: 350 °C, column temperature: initial temperature: 50 °C, initial time: 0 min, heating rate: 9 °C/min, final temperature: 360 °C, final time: 1.5 min, column flow rate (high purity N 2 ): 5 mL/ min, gas (high purity H 2 ): 30 mL/min, combustion-supporting gas (purified Air): 360 mL/min, supplementary gas (high purity N 2 ): 25 mL/min, Sample Volume: 0.2-0.5 mL, operation time: 36 min.

Characterization of the catalyst
Thermogravimetric analysis of coal samples: the raw coal is a Swiss Mettler-Toledo TGA/SDTA851e thermogravimetric analyzer.During the experiment, the carrier gas was selected from high purity N 2 , the gas flow rate was 60 mL/min, the temperature range was 24-900 °C, and the heating rate was 10 °C/min.
The Vario EL III element analyzer (Elementar company in Germany) was used for elemental detection of coal samples.
Specific surface area analysis (BET): The ASP 2460 type surface and pore size analyzer is used to measure the specific surface area of different types of catalysts.
Scanning electron microscope (SEM): JSM-6460LV, working voltage 20 kV, magnification 5000 times.X-ray electron spectroscopy (XPS): XPS was an important tool for analyzing the surface structure and composition of an element, and it can get the composition of the test object.

Coal sample analysis
Industrial and elemental analysis of coal samples Table 2 is the industrial analysis and elemental analysis of coal samples.The ash content of coal sample industrial analysis is determined according to the national standard GB/T212-2008.The coal sample is burned to a constant quality at a temperature of (815 ± 10) ℃, the burning atmosphere is air, and the combustible organic components in the coal are burned; Thermogravimetric analysis is to analyze and determine in the nitrogen atmosphere, and some organic components in coal will not be decomposed; There is a huge difference between the two test conditions, and the ash obtained by the test is very different.The third transition/peak (about 760 °C) in the DTG curve is caused by the pyrolysis and precipitation of asphaltic substances (difficult to decompose macromolecular organic components) in coal, which represents the precipitation temperature of asphaltic substances.It can be seen from Fig. 2 that the weight loss of coal samples is divided into four stages.The first stage is between 24 and 150 °C.This stage is the drying stage, which is mainly the precipitation of moisture and adsorbed gas.The second stage is between 150 and 300 °C, this stage is the preheating stage, no obvious pyrolysis phenomenon occurs, and the TG and DTG curves have no obvious changes.The third stage is between 300 and 700 °C, which is the pyrolysis stage of coal, which is accompanied by the decomposition of functional groups such as phenolic carboxyl groups with poor thermal stability in the molecular structure, and the bridges between aromatic rings in the macromolecular network structure.The breakage of bonds and aliphatic side chains releases a large amount of gaseous hydrocarbons and tar vapors, and the coal sample loses weight rapidly and reaches the maximum weight loss rate.The DT curve of the coal sample drops sharply after 400 °C, and the DTG curve also shows the highest peak of weight loss.The peak temperature of pyrolysis is 450 °C, which is the temperature corresponding to the maximum weight loss temperature.The maximum weight loss temperature reflects the stability of the macromolecular structure of the coal.The lower the peak temperature, the more easily the network structure in the coal is destroyed, and the higher the reactivity of the coal is, the more unstable the structure is during the pyrolysis process 15,16 .

Effect of pyrolysis temperature on tar yield and tar composition
The coal tar yield and tar component distribution at different pyrolysis temperatures are shown in Figs. 3 and  4, respectively.
It can be seen from Fig. 3 that the tar yield increases with the increase of the pyrolysis temperature.When the pyrolysis temperature is 450 °C, the tar yield is 0.19 g, and when the pyrolysis temperature is 600 °C, the tar yield is 0.32 g, which is 68.4% higher than that at 450 °C.The tar produced by the pyrolysis and polycondensation of coal at 600 °C basically reaches the maximum value.It can be seen from Fig. 4 that with the increase of the pyrolysis temperature, the proportion of most of the light fractions in the collected coal tar showed a decreasing trend, and the proportion of washing oil and asphalt showed an increasing trend.It shows that the higher the pyrolysis temperature, the less the total content of light fractions in the collected coal tar and the more pitch.The specific content of each fraction of coal tar also changs at different pyrolysis temperature.The content of light oil, phenol oil and naphthalene oil decreases with the increase of pyrolysis temperature, and the content of anthracene oil first increases and then decreases with the increase of pyrolysis temperature.The pyrolysis temperature has an effect on the degree of coal cracking reaction.When the temperature is low, light components are produced and surface overflow in the coal.With the increase of temperature within 600 °C, the heavier components with larger molecular weights are cracked, the more asphalt products are cracked, and the overall content of light fractions in the collected coal tar decreases.It can be seen from Fig. 5 that when the final temperature of pyrolysis is 600 °C, the tar yield also increases slowly with the increase of constant temperature time.The tar yield of constant temperature pyrolysis for 3 min was 0.32 g, while that of constant temperature pyrolysis for 15 min was 0.37 g.The tar yield of constant temperature pyrolysis for 15 min increased by 15.63% compared with the final pyrolysis temperature of 600 °C.However, the output of pyrolysis gas has been increasing.Since the final temperature of pyrolysis is 600 °C, the secondary degassing is mainly polycondensation, and the volatile components are mainly hydrocarbon gases, hydrogen and carbon oxides, and less tar is produced.It can be seen from Fig. 6 that with the increase of constant temperature pyrolysis time, the proportion of most of the light fractions in the collected coal tar showed a decreasing trend,  With the increase of constant temperature pyrolysis time, the content of anthracene oil first increases and then decreases.The main reason is that the constant temperature pyrolysis time is short, and the tar macromolecules produced cannot be quickly overflowed in the coal.With the prolongation of the constant temperature pyrolysis time, the tar macromolecular substances overflow in the coal, which increases the heavy tar content in the tar components.The light component content is also reduced accordingly.

Effect of alkaline earth metal oxide supported catalyst on tar components
The coal tar yield and tar composition distribution of the alkaline earth metal oxide supported catalyst are shown in Figs.7 and 8, respectively.Figure 7 shows that the addition of alkaline earth metal oxide-supported γ-Al 2 O 3 catalyst can catalyze the pyrolysis of coal and increase tar production 17 .After adding γ-Al 2 O 3 and SrO/γ-Al 2 O 3 , the tar yield increased by 27.27% and 18.18% compared with that of 600 °C, respectively.Therefore, it can be concluded that γ-Al 2 O 3 and SrO/γ-Al 2 O 3 have a significant effect on the tar yield.From Fig. 8, it can be seen that the content of light tar increases and the content of asphalt decreases after adding alkaline earth metal catalyst, which shows that the supported catalyst is beneficial to the production of light tar.The study found that the carrier γ-Al 2 O 3 tar cracking played a catalytic role, increasing the proportion of light oil and phenol oil in the tar.Compared with the pyrolysis of raw coal, the percentage of light oil and phenol oil increases by 23.99% and 45.14%, respectively.The BaO active component tar has the best cracking effect, and its light oil and phenol oil percentages increases the most, increasing by 32.88% and 71.75%, respectively, and the asphalt percentage decreases the most, decreasing by 11.24%.Through the specific surface area test, γ-Al 2 O 3 has a large specific surface area, which is conducive to the dispersion of active metal oxides, and can form smaller crystal grains.surface area, which is favorable for catalytic reactions.

Influence of subgroup metal oxide supported catalyst on tar components
The coal tar yield and tar composition distribution of the subgroup metal oxide-supported catalyst are shown in Figs. 9 and 10, respectively.It can be seen from Fig. 11 that the tar yield is improved after adding Group VIII metal oxide catalyst in the coal pyrolysis process; Cr 2 O 3 /γ-Al 2 O 3 makes the tar yield the highest at 0.71 g, which is 121.82% higher than that of raw coal.It can be seen from Fig. 12 that after adding the Group VIII metal oxide supported catalyst, the content of phenol oil and washing oil in the tar increases significantly, while the content of anthracene oil and pitch decreases.Compared with γ-Al 2 O 3 mixed pyrolysis of Cr 2 O 3 /γ-Al 2 O 3 , it is found that light oil and phenol oil changed by 23.45% and 28.67% respectively; the cracking effect of anthracene oil is obvious, which decreases by 27.12%; Cr 2 O 3 /γ-Al 2 O 3 crackes the most asphalt, and asphalt content decreases by 10.09%.From the increase of light oil and phenol oil and the decrease of anthracene oil and bitumen, Cr

Figure 2 .
Figure 2. TG and DTG curves of coal.

Figure 4 .
Figure 4. Distribution of tar components at different pyrolysis temperatures.

Figure 5 .Figure 6 .
Figure 5. Distribution of coal tar production under different constant temperature pyrolysis times.

Figure 7 .Figure 8 .
Figure 7. Distribution of coal tar production by alkaline earth metal oxide supported catalysts.

Figure 9 .Figure 10 .
Figure 9. Distribution of coal tar production by subgroup metal oxide supported catalysts.
2 O 3 /γ-Al 2 O 3 has the best effect on coal pyrolysis.When the coal sample is mixed with Cr 2 O 3 /γ-Al 2 O 3 for pyrolysis, the coal sample is in contact with the active sites on the surface of Cr 2 O 3 /γ-Al 2 O 3 , which makes the macromolecular chain in the coal break, which promotes the pyrolytic cracking of the coal and generates more heat.solution product.The generated pyrolysis product enters into the interior of Cr 2 O 3 /γ-Al 2 O 3 and contacts with active Cr 2 O 3 , which further breaks the macromolecules to produce small molecular substances, which are easily vaporized and volatilized at higher

Figure 11 .
Figure 11.Distribution of coal tar production by Group VIII metal oxide supported catalysts.
2 O 3 /γ-Al 2 O 3 catalyst with 5% loading was removed after cooling to room temperature.Co 2 O 3 /γ-Al 2 O 3 and NiO/γ-Al 2 O 3 catalysts were prepared by the same method.Preparation of VIII metal oxides catalyst supported on γ-Al 2 O 3 : The equal volume impregnation method was used.3 g γ-Al 2 O 3 was immersed in 5% (CH 2 COO) 2 Zn solution for 24 h and then removed and placed in a muffle oven at 500 °C for 4 h.The ZnO/γ-Al 2 O 3 catalyst with 5% loading was removed after cooling to room temperature.Mo 2 O 3 /γ-Al 2 O 3 , Cr 2 O 3 /γ-Al 2 O 3 and MnO 2 /γ-Al 2 O 3 catalysts were prepared by the same method.

Table 1 .
Boiling points range for classifying tar fractions.

Table 2 .
IndustrialAnalysis of coal samples N 2 atmosphere, heating rate 10 °C/min, weight loss curve (TG) and weight loss rate curve (DTG) are shown in Fig.2.